1. Field of the Invention
The present invention relates to apparatuses for optically reading a target to which optically readable information, such as an information code, for example, a barcode or a two-dimensional code, is attached.
2. Description of the Related Art
Handheld optical information readers aim at reading information codes optically readable, such as barcodes, two-dimensional codes, or other similar codes. In this specification, a target itself or a target to which optically readable information is attached is collectively referred to as the “target”.
For improving the usability of handheld optical information readers, handheld optical information readers that can read an information code positioned at a distance therefrom have been provided.
The handheld optical information reader has a handheld body case provided at its one end portion with a reading window. In addition, the handheld optical information reader also has a photodetector, such as a CCD (Charge-Coupled Device) area sensor, an imaging optics with an imaging lens, and a reading unit composed of a light illuminating device, such as an LED (light emitting diode). The photodetector, the imaging unit, and the reading unit are installed in the body case, respectively.
In order to align the reading window (the photodetector) with the target, handheld optical information readers have been commonly provided with a marker beam irradiating unit using a laser diode (LD), an LED, or other similar light emitting devices. The marker beam irradiating unit is operative to irradiate a marker beam onto the target for indicating a reader's reading position, such as a field of view (FOV) of the photodetector, and/or the center position of the FOV, onto the target.
An example of such marker beam irradiating units is disclosed in Japanese non-examined Patent Publication No. H9-201689. The disclosed marker beam irradiating unit uses an LD, as a light source, capable of emitting a marker beam with high-visibility, and a slit plate for forming a predetermined shaped beam pattern on the target.
Specifically, as illustrated in FIGS. 9A, 9B, and 10, a marker beam irradiating device 1 is provided with an LD (laser diode) 2, and a slit plate 3 opposite to the laser-beam emitting plane of the LD with a predetermined interval. In addition, the marker beam irradiating device 1 is provided with an imaging lens 4 coaxially arranged with respect to the LD to be opposite to the slit plate 3 with a predetermined interval.
As illustrated in FIGS. 9B and 11, the slit plate 3 has a metal thin-plate, and a plurality of slits 3a formed therethrough. The shape of each slit 3a and arrangement of the slits 3a correspond to a desirable beam pattern of a marker beam M10. For example, the beam pattern of the marker beam M 10 consists of four L-shaped pattern elements corresponding to the four corner portions of the field of view of a CCD area sensor as a photodetector of an optical information reader. In addition, the beam pattern of the marker beam M10 consists of a cross pattern element indicating the center of the field of view.
Specifically, in the marker beam irradiating device 1, a laser beam emitted from the laser diode 2 is entered the slit plane 3, so that beamlets passing through the slits 3a of the slit plane 3 are irradiated as the marker beam M10 with the desirable beam pattern onto a target R through the imaging lens 4 (see FIG. 10).
In the structure of the marker beam irradiating device 1, as illustrated in FIG. 10, a suitable relationship between the distance “a” between the principal point of the imaging lens 4 along the optical axis direction and the distance “b” between the laser beam emitting position of the LD 2 and the principal point of the imaging lens therealong has been represented as the following formula (Lens formula):1/a+1/b=1/f
where “f” represents the focal distance of the imaging lens 4.
In the structure of the marker beam irradiating device 1, however, as illustrated in FIG. 11, the laser beam emitted from the LD is diffused to have a substantially horizontally prolate ellipsoid profile L0 in its lateral cross section. This may cause loss of the amount of laser beam when it passes through the slits 3a of the slit plate 3, resulting in that the amount of brightness of the marker beam may be insufficient.
In order to prevent the loss of the amount of laser beam, it is to be considered to collect the diffused laser beam in a substantially circular form in its lateral cross section and to cause the collected laser beam to be incident into the slit plate 3.
Specifically, as illustrated in FIG. 12, a marker beam irradiating device 5 is provided with a collective lens 6 coaxially arranged between the LD 2 and the slit plate 3, in addition to the structure of the device 1 in FIG. 9A. The collective lens 6 is operative to collect the laser beam emitted from the LD 2 in a substantially circular form in its lateral cross section.
The structure of marker beam irradiating device 5, however, may cause the area of slit plate 3 to be excessively small. This may make it difficult to form the slits 3a through the thin-plate, or to pass the collected beam through the slits 3a because of the excessively thin width of each slit 3a. The later problem may cause the brightness of marker beam M10 on the target R to decrease, and a fringe pattern (diffraction fringe pattern) to appear on the target R.
For avoiding the decrease of the slit plate's area, as illustrated in FIG. 13, a marker beam irradiating device 7 is designed such that the LD 2 and the collective lens 6 are arranged to be sufficiently away from each other along the optical axis direction. This structure can ensure enough area for the slit plate 3 to allow the collected laser beam to pass through the slits 3a. 
The structure of marker beam irradiating device 7, however, may cause its length along the optical axis direction to increase, in other words, its size to increase. This may deteriorate the installability of the device 7, making it difficult to install the device 7 in handheld optical information readers.